Protein C is a serine protease and naturally occurring anticoagulant that plays a role in the regulation of hemostasis by inactivating Factors Va and VIIIa in the coagulation cascade. Human protein C circulates as a 2-chain zymogen which is activated in vivo by thrombin and thrombomodulin on phospholipid surfaces resulting in activated protein C (aPC).
Blood coagulation is a highly complex process regulated by the balance between pro-coagulant and anticoagulant mechanisms. This balance determines a condition of either normal hemostasis or abnormal pathological thrombus formation leading to events such as stroke, myocardial infarction and venous thrombosis. Two major factors control this balance, the generation of fibrin and the activation and subsequent aggregation of platelets. A critical factor controlling both processes is the generation of the enzyme thrombin, which occurs following activation of the clotting cascade. Thrombin is a pro-coagulant enzyme that aggregates platelets and converts circulating fibrinogen to insoluble fibrin, resulting in the formation of a blood clot. Thrombin also functions as a potent anticoagulant since it activates protein C zymogen to activated protein C, which in turn inhibits the generation of thrombin. Thus, through the feedback regulation of thrombin generation, aPC functions as perhaps the most important down-regulator of blood coagulation resulting in protection against thrombosis.
The critical role of protein C in controlling hemostasis is exemplified by the increased rate of thrombosis in heterozygous deficiency, protein C resistance (e.g., due to the common Factor V Leiden mutation) and the fatal outcome of untreated homozygous protein C deficiency. Human activated protein C, both plasma-derived and recombinant, have been shown to be effective and safe antithrombotic agents in a variety of animal models of both venous and arterial thrombosis.
In current clinical practice, platelet inhibition, e.g., using aspirin (ASA), is well documented for efficacy in both prevention and treatment of thrombotic disease. Moreover, in conditions such as myocardial infarction and stroke, platelet inhibition has become the standard of care. However, the use of antiplatelet agents such as ASA increases the risk of bleeding, which limits the dose of the agent and duration of treatment. To block the effect of thrombin in fibrin formation, heparin remains the standard anticoagulant in the acute care setting. However, heparin has a narrow therapeutic index and is associated with significant bleeding risk especially in combination with antiplatelet agents.
Combination therapy with aspirin and a synthetic thrombin inhibitor, tissue plasminogen activator, or a monoclonal antiplatelet glycoprotein IIb/IIIa antibody has been studied in a canine coronary artery thrombosis model [Yasuda, et al., J Am Coll Cardiol., 16:714-22 (1990)]. Aspirin in combination with these agents prolonged the bleeding time and did not prevent reocclusion of the coronary artery. In addition, combination therapy has been proposed for aPC with thrombolytic agents such as tissue plasminogen activator, urokinase, or streptokinase [Griffin, et al., U.S. Pat. No. 5,350,578]. However, these combinations have not proved successful. Thus, there remains the need to identify an effective therapy for treating thrombotic disorders.
The present invention is the first to describe the combination of aPC with antiplatelet agents in the treatment of thrombosis. Accordingly, the present invention provides the use of aPC in combination with antiplatelet agents for the treatment of thrombotic disorders. This combination therapy results in enhanced efficacy in a variety of thrombotic disorders including, but not limited to, stroke, myocardial infarction, unstable angina, abrupt closure following angioplasty or stent placement, and thrombosis as a result of peripheral vascular surgery. Furthermore, the combination of aPC and antiplatelet agents results in a synergy that will allow the reduction of the dosages of both aPC and the antiplatelet agents. The reduction of the dosages of the agents in combination therapy in turn results in reduced side effects such as increased bleeding liability often observed in combination anticoagulant/antiplatelet therapy.